(a) Under the Clean Air Act, an agency of two or more municipalities located in the same State or in different States and having substantial powers or duties pertaining to the prevention and control of air pollution.

(b) Under the Resource Conservation and Recovery Act, an agency established by two or more municipalities with responsibility for planning or administration of solid waste.

(c) In all other cases, an agency of two or more municipalities having substantial powers or duties pertaining to the control of pollution.